Several factors:
1) Depends on how long your flight is. Short flights (under 2 hours) i'll just wear my usual full makeup and wear stylish yet comfy clothes.
2) Depends on the country you're going to, sometimes it's safer to dress down, in order to attract less attention. i'll be visiting my homeland soon and will consciously be totally dressing down.
3) Depends on what 'class' you're in:
First class, i'll make more of an effort to be a bit more stylish, but i'll never wear heels or stockings - ever... they're not comfortable and if there's a fire, if you wear stockings they'll melt on your skin!
Economy/Chartered flights/and for longer flights, i'll be comfortable - usually i'll wear comfy jeans, sneaks, ankle socks, short or long-sleeve tee w/ a zip-cardigan - essential for me.. always find myself freezing on the plane. When i head off to the loo, i fold the hem of my jeans up as i find the loo in economy can get dodgy. Overall i stay away from synthetics as i do have this fire fear thing (after seeing a program) anything w/ polyester or nylon will melt and stick on you. Another alternative is a simple black turtleneck or black tee + black pants, black zip-boots (or sandals) + a nice coloured pashmina on top.
Makeup - basics - i'll wear more moisturizer than usual + either concealer alone or concealer + a sheer foundation. A little eyeshadow + lipgloss. i carry a homemade spritzer of rosewater + glycerin, and reapply moisturizer. As we're about to land, i reapply concealer/foundation, add eyeliner, curl my lashes, add mascara, apply lipstick and wear blush/bronzer.... and of course, fix my hair.
Next week i'll be applying your tips... using earplugs for sure.. maybe i'll try Visine Real Tears to rehydrate my poor eyes.